The contamination of surface and near-surface soils has become a matter of great concern in many locations. Soil may become contaminated with chemical, biological, and/or radioactive contaminants. Material spills, leaking storage vessels, and landfill seepage of improperly disposed materials are just a few examples of the many ways in which soil may become contaminated. If left in place, many of these contaminants will find their way into aquifers, air, or into the food supply, and could become public health hazards.
There are many proposed methods for removal of surface contaminants, such as excavation followed by incineration, in situ vitrification, biological treatment, chemical additives for deactivation, radiofrequency heating, etc. U.S. Pat. No. 5,337,684 for instance describes a method and apparatus for removing vaporizable contaminants from flowable materials such as liquids, sludge or soil. The contaminated material is removed from its site by means of a conveyor, and further introduced into a treatment vessel, where it will be heated such that contaminants in the soil will be vaporized, after which this obtained contaminant vapor is further incinerated and decontaminated. Although successful in some applications, these methods can be very expensive and are not practical if many tons of soil must be treated.
A process that may be used to remove contaminants from subsurface soil is a soil vapor extraction process. In such process a vacuum is applied to the soil to draw air and vapor through subsurface soil. The vacuum may be applied at a soil/air interface, or the vacuum may be applied through vacuum wells placed within the soil. The air and vapor may entrain and carry volatile contaminants towards the source of the vacuum. Off-gas removed from the soil by the vacuum which includes contaminants that were within the soil is then transported to a treatment facility wherein it is processed to eliminate, or reduce contaminants to acceptable levels.
In situ thermal desorption may be used to increase the effectiveness of a soil vapor extraction process. In situ thermal desorption involves in situ heating of the soil to raise the temperature of the soil while simultaneously removing off-gas from the soil. Heat added to contaminated soil may raise the temperature of the soil above vaporization temperatures of contaminants within the soil and cause the contaminants to vaporize. A vacuum applied to the soil allows dragging the vaporized contaminant out of the soil.
One method of heating a soil containing contaminants comprises the injection of a heated fluid into the soil.
Such method is for instance described in U.S. Pat. No. 6,000,882. The herein described method consists of introducing a system of perforated pipes into the soil. A stream of hot air is sent through the pipes. The hot air is injected into the soil through perforations in the pipes at the level of the pipe perforations. A contaminant vapor is formed in the soil, which may be removed from the soil through the perforations in the pipes and disposed to an off-gas treatment unit.
A similar system is described in U.S. Pat. No. 5,228,804. Herein two series of perforated pipes are introduced in a heap of contaminated soil that has been excavated. One series is applied at the heap basis and is suitable for injecting hot air through the pipe perforations into the soil. Another series of pipes is applied at the top of the soil heap and is suitable for dragging the contaminant gases together with the percolating hot air out of the heated soil. Besides the need to use at least two series of different acting pipes, which have in addition to be positioned towards each other in a well-defined way, the disclosed method further has the disadvantage that the soil heap always needs to be covered with an isolating blanket or the like, to avoid dissipation of contaminant gasses into the atmosphere. Furthermore, the described method is not suitable for in situ soil treatment, and energetic unfavorable, since a high input of energy is required for effectively heating the soil.
Another major draw back of the above-described type of method however, is that hot air injection into the soil is prone to create vapor flow paths in the soil. Also, percolation of hot air through the soil may be hampered by the soil type, such as e.g. clay. As a consequence, the hot air is not homogenously distributed in the contaminated soil, but rather accumulates at its injection level in the soil; i.e. in and around the pipe perforations.
Another way of heating a soil consists of heating a soil by thermal conduction. Thermal conductive heating of a contaminated soil in combination with the removal of contaminant gases from the soil using a vapor extraction system is old in the art.
Thermal blankets and/or ground heaters that are placed on top of the contaminated soil have been applied for conductively heating a soil. U.S. Pat. No. 5,169,263, for instance, describes a decontamination system wherein the contaminated soil is covered with a heater element. The heat generated at the soil surface is conducted and convected downwardly into the soil. As the soil temperature rises, contaminants evaporate and flow towards perforated pipes provided in the contaminated soil. The flow of contaminant vapor through the pipes is encouraged by pressure reducing means, typically a vacuum pump, acting in cooperation with the pipes to lower the pressure at or around the pipes. A drawback of such method however is that permeability of the soil may limit the effectiveness of the heating process such that the heat is not homogenously distributed in the contaminated soil.
Alternatively, systems have been described wherein thermal conductive heating of the soil may include resistively (electrically) heating a well casing, which conductively heats the surrounding soil. Coincident or separate source vacuum may be applied.
In U.S. Pat. No. 5,244,310, for instance, a method and system for remediation of contaminated soil is described, wherein a frame is applied to which a plurality of heating elements and vapor collecting elements are connected. The heating elements are heated by electrical power supplied from a power supply, and the heat is conducted and convected to the soil surrounding the elements. A vacuum extraction system is connected to the vapor collecting elements and puts the elements under a negative pressure, such that contaminant vapor can be collected and withdrawn out of the soil via the vapor collecting elements.
From US 2002/0018697 a soil remediation system is known wherein heat may be transferred to the soil from resistively heated bare metal heater elements. The heater elements may be placed within the soil. The system further comprises a vapor collection system that consists of a plurality of pipes connected to a vacuum system for providing a vacuum to the soil and for removing off-gas from the soil.
U.S. Pat. No. 5,318,116 describes in situ thermal desorption systems and processes for treating contaminated subsurface soil with thermal conductive heating applied to soil from electrically heated heater wells provided in a casing. The heater wells are placed in the contaminated soil where they conductively heat the soil to elevated temperatures. The heater wells are connected to a vacuum manifold for collection of the contaminant vapors. The wells are permeable to the vapors which emanate from the soil when heated and are drawn towards the heater wells by the imposed vacuum.
A common drawback of the above-mentioned methods however is that they are relatively inefficient from an energetic point of view. In these methods, a contaminated soil is heated, vaporized soil contaminants are extracted out of the soil and decomposed or destroyed on site e.g. in a thermal treatment system. However, heating of the soil as well as thermally treating the soil contaminants extracted out of the soil are both processes which require the input of a substantial amount of energy. The above-mentioned methods thus require a large input of energy and therefore bring along large operating expenses.
The present invention aims to provide a solution to the above-mentioned problem by providing a method and system for cleaning a soil containing contaminants which is more efficient from an energetic point of view. In particular, the present invention aims to provide a nearly closed loop method and system for cleaning a soil containing contaminants wherein the energy which is obtained by thermally treating the soil contaminants is at least partly recuperated and re-used.